1. Field of the Invention
The present invention broadly relates to novel methods of producing self-supporting ceramic materials and to the products thereof exhibiting superior properties as compared with conventional or known ceramic bodies. In its more specific aspects, this invention relates to methods of producing self-supporting ceramic bodies and products thereof from a parent metal by oxidation of the molten parent metal having applied to its surface one or more dopants, thereby resulting in a polycrystalline oxidation reaction product, useful as a ceramic material.
2. Background
Ceramics have, in recent years, been increasingly considered as candidates for structural applications historically served by metals. The impetus for this substitution has been the superior properties of ceramics, such as corrosion resistance, hardness, modulus of elasticity, and refractory capabilities when compared with metals, coupled with the fact that the engineering limits of performance of many modern components and systems are now gated by these properties in conventionally employed materials. Examples of areas for such prospective substitution include engine components, heat exchangers, cutting tools, bearings and wear surfaces, pumps, and marine hardware.
However, the key to enabling the substitution of ceramics for metals in such structural applications has been the cost-effective development of improved strength and fracture toughness characteristics sufficient to withstand tensile loading, vibration, and impact. Current efforts to produce high strength, reliable monolithic ceramics have focused upon improved powder processing technologies, and although these efforts have resulted in improvements in ceramic performance they are also complicated and generally less than cost-effective. The emphasis in such conventional powder processing technologies has been in two areas: (1) improved methods of producing ultra-fine, uniform powder materials using sol-gel, plasma, and laser techniques, and (2) improved methods of densification and compaction, including superior sintering techniques, hot pressing and hot isostatic pressing. The object of these efforts is to produce dense, fine-grained, flaw-free microstructures and, in fact, some improvement in performance capabilities in ceramics has been attained in these areas. However, these developments have generally resulted in dramatic increases in the cost of producing ceramic structures.
One limitation in ceramic engineering which is aggravated by modern ceramic processing is scaling versatility. Conventional processes aimed at densification (i.e., removal of voids between powder particles) are incompatible with large, one-piece structural application possibilities for ceramics, such as monolithic furnace liners, pressure shells, boiler and superheater tubes, etc. Several practical problems are encountered in the conventional processing of ceramic parts with an increase in part size. The problems include, for example, increased process residence times, stringent requirements for uniform process conditions over a large process volume, cracking of parts due to nonuniform densification if conditions are not sufficiently uniform, excessive compaction forces and die wall thickness dimensions if hot pressing is used, and excessive pressure vessel costs due to internal volume and wall thickness requirements in the case of hot isostatic pressing.
Occasionally in the past, oxidation of metals has been contemplated as a conceptually attractive approach to the formation of an oxide-type ceramic body. In this regard, it may be noted that metals oxidize generally in one of four modes. Certain metals oxidize when exposed to an oxidizing environment to form an oxidation reaction product which flakes, spalls or is porous, such that the metal surface is continually exposed to the oxidizing environment. In such a process, a free-standing or self-supporting body is not formed as the metal oxidizes, but rather, a mass of flakes or particles is formed. Iron, for example, reacts with oxygen so as to oxidize in such a manner. Certain other metals such as aluminum, magnesium, chromium, nickel or the like oxidize in such a manner as to form a relatively thin, protective oxidation reaction product skin which transports either oxidant or metal (or both) at such a low rate that the underlying metal is effectively protected from further oxidation. This mechanism does not yield a free-standing structure of a thickness sufficient to exhibit any significant structural integrity. Still other metals are known to form a solid or liquid oxidation reaction product film which does not protect the underlying parent metal because such reaction products permit the transport of oxidant therethrough. While an oxygen-permeable film may retard the oxidation rate of the underlying metal, the metal itself is not totally protected by the film due to oxidant-permeability thereof. An example of this latter type of oxidation occurs in the case of silicon which, when exposed to air at elevated temperatures, forms a glassy skin of silicon dioxide which is permeable to oxygen. Typically, these processes do not occur at nearly fast enough rates to produce a useful thickness of ceramic material. Finally, other metals form oxidation reaction products which volatize and continually expose fresh metal to oxidation. Tungsten is an example of a metal which oxidizes in this manner when reacted with oxygen at high temperatures to form WO.sub.3.
In an attempt to produce thicker ceramic oxide layers, fluxes have been added to the surfaces of metals such as aluminum and magnesium to dissolve or break up their oxides and render them susceptible to oxygen or metal transport so that thicker oxide skins can be produced. However, the ability to form free-standing ceramic structures by such a technique has heretofore been limited to thin sections of relatively limited strength.
The prior art shows the use of fluxes on metal powders to dissolve or break up their oxidation reaction products to facilitate oxidation of their surfaces in admixture with other particulates to yield intrinsically porous, low strength ceramics, as described in U.S. Pat. No. 3,255,027 to H. Talsma and U.S. Pat. No. 3,299,002 to W. A. Hare. Similar methods may be used to produce thin-walled Al.sub.2 O.sub.3 refractory structures (U.S. Pat. No. 3,473,987 to D. R. Sowards and U.S. Pat. No. 3,473,938 to R. E. Oberlin) or thin walled hollow refractory particles (U.S. Pat. No. 3,298,842 to L. E. Seufert). However, a characteristic of such processes is the limited thickness of oxidation reaction product which is formed, apparently because the effect of a fluxing agent is of relatively short duration such that the oxidation reaction product reverts to a slow-growing, protective character after only a limited amount of growth. Increasing the flux concentration to promote thicker ceramic skin growth results in a lower strength, less refractory, lower hardness product and, therefore, is counter-productive.
One technique which has been successfully employed to create freestanding ceramics by the oxidation of metals involves an oxidation/reduction or "redox" type reaction. It has long been known that certain metals will reduce other metal oxides to form a new oxide and, also, a reduced form of the original oxide. The use of such redox-type reactions to produce ceramic materials is described in U.S. Pat. No. 3,437,468 to L. E. Seufert and U.S. Pat. No. 3,973,977 to L. E. Wilson. The primary disadvantage of the redox-type reactions described in these patents is their inability to produce a singular, hard, refractory oxide phase as a reaction product, but rather the products contain multiple oxide phases which degrade the hardness, modulus of rupture and wear resistance characteristics.